Insulin receptor autophosphorylation or phosphorylation of endogenous substrates by the receptor kinase appear to be necessary for receptor signal transduction. While our knowledge of phosphorylation reactions involving the receptor is substantial, little is known about the equally important dephosphorylation reactions which are essential for the reversibility of the hormonal signal in target tissues and may play a role in modulating the sensitivity and/or responsiveness of target tissues to insulin. The objective of this research project is to provide further experimental support for the hypothesis that a member of the transmembrane or receptor-like family of phosphotyrosine phosphatases (PTPases) exerts selective, negative regulatory control on insulin receptor signaling. Two novel experimental models will be used in these investigations. First is the rat hepatoma cell line, McA-RH7777, which is stably transfected with antisense sequences to the transmembrane PTPase LAR. Reduction in LAR expression by 63% is associated with increased receptor autophosphorylation, receptor tyrosine kinase activity, and increased PI3- kinase activation. Second is the insulin responsive multiple myeloma cell line U266 from which two stable sub-populations have been isolated; one which expresses the transmembrane PTPase CD45 and another which does not. Loss of CD45 expression is associated with increased insulin receptor signal transduction (receptor autophosphorylation and PI3-kinase activation). These two model systems will be exploited in order to investigate the role for transmembrane PTPases in regulating insulin signal transduction. With the long term goal of defining regulation of insulin receptor signal transduction in target cells, the following specific aims will be pursued. First, characterize the regulatory role of the transmembrane PTPase LAR on the IR signal transduction pathway in the rat hepatoma cell line, McA- RH7777, in which expression of the PTPase LAR is suppressed by stable transfection of antisense sequences. Second, characterize the regulatory role of the transmembrane PTPase CD45 on the IR signal transduction pathway in U266 myeloma cells and compare and contrast results to those of LAR. Third, investigate the mechanisms (direct versus indirect) by which the transmembrane PTPases LAR and CD45 interact with the insulin receptor. Together these investigations have as their objective to establish basic principles by which transmembrane PTPases and the insulin receptor interact. Insulin receptor dephosphorylation reactions mediated by PTPases may play a vital role in metabolic regulation. An understanding of these reactions is essential for a complete understanding of normal and abnormal (ie. insulin resistance) tissue responses to insulin.